Quantum cascade laser (QCL) based spectroscopic techniques are useful for the quantitative detection and monitoring of molecular trace gas species in the mid-infrared spectral region. Sensitive and selective trace gas detection can be realized by targeting distinct and strong absorption lines in the mid-infrared region with QCLs in this spectral region. Basic concepts of QCL based spectroscopic techniques are described. QCL based sensor systems are of interest for biomedical, environmental, and industrial applications utilizing various spectroscopic methods. This thesis describes the design, development and performance of three sensor systems. A tunable laser absorption spectroscopy (TLAS) based sensor system for nitric oxide detection was designed, which achieved a minimum detection limit of 505 pptv with a 1 second data acquisition time. A quartz enhanced photoacoustic spectroscopy (QEPAS) based H2O2 sensor system was implemented with a minimum detection limit of 12 ppbv with a 100 sec averaging time corresponding to a normalized noise equivalent absorption (NNEA) coefficient of 4.6×10-9 cm-1W/Hz-1/2¬. In addition, a QEPAS sensor system with capability of detecting both methane (CH4) and nitrous oxide (N2O) was developed and investigated. The measured minimum detection limits with a 1 second acquisition time of CH4 and N2O are 13 ppbv and 6 ppbv, respectively. The developed CH4/N2O QEPAS sensor was installed in a mobile laboratory operated by Aerodyne, Inc during a 2013 NASA DISCOVER_AQ field campaign to perform atmospheric CH4 and N2O measurements adjacent to two urban waste disposal sites in the Greater Houston urban area.